fix(transport): avoid cross-task stderr cancel scope

[MukundaKatta]

Summary

• replace the manually-entered anyio stderr task group with an asyncio task
• make transport shutdown safe even when close() runs from a different task context
• add a regression test that closes the transport from another task

Testing

• python3 -m py_compile src/claude_agent_sdk/_internal/transport/subprocess_cli.py tests/test_transport.py
• python3 -m pytest tests/test_transport.py -k "connect_close or different_task_context" (fails in this environment because the test import path is missing the mcp dependency)